In large industrial scale systems, efficiency may be a critical aspect of operations. Even small improvement of system efficiency can lead to significant cost savings; likewise, loss of efficiency may lead to increased costs or even system failure. Chillers represent a significant type of industrial system, since they are energy intensive to operate, and are subject to variation of a number of parameters which influence system efficiency and capacity.
The vast majority of mechanical refrigeration systems operate according to similar, well known principles, employing a closed-loop fluid circuit through which refrigerant flows, with a source of mechanical energy, typically a compressor, providing the motive forces for pumping heat from an evaporator to a condenser. In a chiller, water or brine is cooled in the evaporator for use in a process. In a common type of system, discussed in more detail below, the evaporator is formed as a set of parallel tubes, forming a tube bundle, within a housing. The tubes end on either side in a separator plate. The water or brine flows through the tubes, and the refrigerant is separately provided on the outside of the tubes, within the housing.
The condenser receives hot refrigerant gas from the compressor, where it is cooled. The condenser may also have tubes, which are, for example, filled with water which flows to a cooling tower. The cooled refrigerant condenses as a liquid, and flows by gravity to the bottom of the condenser, where it is fed through a valve or orifice to the evaporator.
The compressor therefore provides the motive force for active heat pumping from the evaporator to the condenser. The compressor typically requires a lubricant, in order to provide extended life and permit operation with close mechanical tolerances. The lubricant is an oil which miscible with the refrigerant. Thus, an oil sump is provided to feed oil to the compressor, and a separator is provided after the compressor to capture and recycle the oil. Normally, the gaseous refrigerant and liquid lubricant are separated by gravity, so that the condenser remains relatively oil free. However, over time, lubricating oil migrates out of the compressor and its lubricating oil recycling system, into the condenser. Once in the condenser, the lubricating oil becomes mixed with the liquefied refrigerant and is carried to the evaporator. Since the evaporator evaporates the refrigerant, the lubricating oil accumulates at the bottom of the evaporator.
The oil in the evaporator tends to bubble, and forms a film on the walls of the evaporator tubes. In some cases, such as fin tube evaporators, a small amount of oil enhances heat transfer and is therefore beneficial. In other cases, such as nucleation boiling evaporator tubes, the presence of oil, for example over 1%, results in reduced heat transfer. See, Schlager, L. M., Pate, M. B., and Berges, A. E., “A Comparison of 150 and 300 SUS Oil Effects on Refrigerant Evaporation and Condensation in a Smooth Tube and Micro-fin Tube”, ASHRAE Trans. 1989, 95(1):387-97; Thome, J. R., “Comprehensive Thermodynamic Approach to Modelling Refrigerant-Lubricating Oil Mixtures”, Intl. J. HVAC&R Research (ASHRAE) 1995, 110-126; Poz, M. Y., “Heat Exchanger Analysis for Nonazeotropic Refrigerant Mixtures”, ASHRAE Trans. 1994, 100(1) 727-735 (Paper No. 95-5-1).
A refrigeration system is typically controlled at a system level in one of two ways: by regulating the temperature of the gas phase in the top of the evaporator (the superheat), or by seeking to regulate the amount of liquid (liquid level) within the evaporator. As the load on the system increases, the equilibrium within the evaporator changes. Higher heat load will increase temperatures in the headspace. Likewise, higher load will boil more refrigerant per unit time, and lead to lower liquid levels.
For example, U.S. Pat. No. 6,318,101, expressly incorporated herein by reference, relates to a method for controlling an electric expansion valve based on cooler pinch and discharge superheat. This system seeks to infer the level of refrigerant in the evaporator and control the system based thereon, while preventing liquid slugging. A controlled monitors certain variables which are allegedly used to determine the optimal position of the electronic expansion valve, to optimize system performance, the proper discharge superheat value, and the appropriate refrigerant charge. See also, U.S. Pat. No. 6,141,980, expressly incorporated herein by reference.
U.S. Pat. No. 5,782,131, expressly incorporated herein by reference, relates to a refrigeration system having a flooded cooler with a liquid level sensor.
Each of these strategies provides a single fixed setpoint which is presumed to be the normal and desired setpoint for operation. Based on this control variable, one or more parameters of operation are varied. Typically, a compressor will either have a variable speed drive or a set of variable angle vanes which deflect gaseous refrigerant from the evaporator to the compressor. These modulate the compressor output. Additionally, some designs have a controllable expansion valve between the condenser and evaporator. Since there is a single main control variable, the remaining elements are controlled together as an inner loop to maintain the control variable at the setpoint.
Typical refrigerants are substances that have a boiling point (at the operating pressure) below the desired cooling temperature, and therefore absorb heat from the environment while evaporating (changing phase) under operational conditions. Thus, the evaporator environment is cooled, while heat is transferred to another location, the condenser, where the latent heat of vaporization is shed. Refrigerants thus absorb heat via evaporation from one area and reject it via condensation into another area. In many types of systems, a desirable refrigerant provides an evaporator pressure as high as possible and, simultaneously, a condenser pressure as low as possible. High evaporator pressures imply high vapor densities, and thus a greater system heat transfer capacity for a given compressor. However, the efficiency at the higher pressures is lower, especially as the condenser pressure approaches the critical pressure of the refrigerant.
The overall efficiency of the refrigeration system is influenced by the heat transfer coefficients of the respective heat exchangers. Higher thermal impedance results in lower efficiency, since temperature equilibration is impaired, and a larger temperature differential must be maintained to achieve the same heat transfer. The heat transfer impedance generally increases as a result of deposits on the walls of the heat exchangers, although, in some cases, heat transfer may be improved by various surface treatments and/or an oil film.
Refrigerants must satisfy a number of other requirements as best as possible including: compatibility with compressor lubricants and the materials of construction of refrigerating equipment, toxicity, environmental effects, cost availability, and safety. The fluid refrigerants commonly used today typically include halogenated and partially halogenated alkanes, including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HFCFs), and less commonly hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs). A number of other refrigerants are known, including propane and fluorocarbon ethers. Some common refrigerants are identified as R11, R12, R22, R500, and R502, each refrigerant having characteristics that make them suitable for different types of applications.
In an industrial chiller, the evaporator heat exchanger is a large structure, containing a plurality of parallel tubes in a bundle, within a larger vessel comprising a shell. The liquid refrigerant and oil form a pool in the bottom of the evaporator, boiling and cooling the tubes and their contents. Inside the tubes, an aqueous medium, such as brine, circulates and is cooled, which is then pumped to another region where the brine cools the industrial process. Such an evaporator may hold hundreds or thousands of gallons of aqueous medium with an even larger circulating volume. Since evaporation of the refrigerant is a necessary part of the process, the liquid refrigerant and oil must fill only part of the evaporator.
It is also known to periodically purge a refrigeration or chiller system, recycling purified refrigerant through the system to clean the system. This technique, however, generally permits rather large variance in system efficiency and incurs relatively high maintenance costs. Further, this technique generally does not acknowledge that there is an optimum (non-zero) level of oil in the evaporator and, for example, the condenser. Thus, typical maintenance seeks to produce a “clean” system, which may be suboptimal, subject to incremental changes after servicing. Refrigerant from a refrigeration system may be reclaimed or recycled to separate oil and provide clean refrigerant, in a manual process that requires system shutdown.
U.S. Pat. No. 6,260,378, expressly incorporated herein by reference, relates to a refrigerant purge system, in particular to control removal of non-condensable gases.
The oil in the evaporator tends to accumulate, since the basic design has no inherent path for returning the oil to the sump. For amounts in excess of the optimum, there are generally reduced system efficiencies resulting from increasing oil concentration in the evaporator. Thus, buildup of large quantities of refrigerant oil within an evaporator will reduce efficiency of the system.
In-line devices may be provided to continuously remove refrigerant oil from the refrigerant entering the evaporator. These devices include so-called oil eductors, which remove oil and refrigerant from the evaporator, returning the oil to the sump and evaporated refrigerant to the compressor. The inefficiency of these continuous removal devices is typically as a result of the bypassing of the evaporator by a portion of the refrigerant, and potentially a heat source to vaporize or partially distill the refrigerant to separate the oil. Therefore, only a small proportion of the refrigerant leaving the condenser may be subjected to this process, resulting in poor control of oil level in the evaporator and efficiency loss. There is no adequate system for controlling the eductor. Rather, the eductor may be relatively undersize and run continuously. An oversize eductor would be relatively inefficient, since the heat of vaporization is not efficiently used in the process.
Another way to remove oil from the evaporator is to provide a shunt for a portion of mixed liquid refrigerant and oil in the evaporator to the compressor, wherein the oil is subject to the normal recycling mechanisms. This shunt, however, may be inefficient and is difficult to control. Further, it is difficult to achieve and maintain low oil concentrations using this method.
U.S. Pat. No. 6,233,967, expressly incorporated herein by reference, relates to a refrigeration chiller oil recovery system which employs high pressure oil as an eductor motive fluid. See also, U.S. Pat. Nos. 6,170,286 and 5,761,914, expressly incorporated herein by reference.
In both the eductor and shunt, as the oil level reaches low levels, e.g., about 1%, 99% of the fluid being separate is refrigerant, leading to significant loss of process efficiency.
It is noted that it is difficult to accurately sample and determine the oil concentration in the evaporator. As the refrigerant boils, oil concentration increases. Therefore, the oil concentration near the top of the refrigerant is higher than the bulk. However, as the boiling liquid churns, inhomogeneities occur, and accurate sampling becomes difficult or impossible. Further, it is not clear that the average bulk oil concentration is a meaningful control variable, apart from the effects of the oil on the various components. Since it is difficult to measure the oil concentration, it is also difficult to measure the amount of refrigerant in the evaporator. A difficulty of measurement of the amount of refrigerant is compounded by the fact that, during operation, the evaporator is boiling and froths; measuring the amount during a system shutdown must account for any change in distribution of the refrigerant between the other system components.
It is known that the charge conditions of a chiller may have a substantial effect on both system capacity and system operating efficiency. Obviously, if the amount of liquid refrigerant in the evaporator is insufficient, the system cannot meet its cooling needs, and this limits capacity. Thus, in order to handle a larger heat load, a greater quantity of refrigerant, at least in the evaporator, is required. However, in typical designs, by providing this large refrigerant charge, the operating efficiency of the system at reduced loads is reduced, thus requiring more energy for the same BTU cooling. Bailey, Margaret B., “System Performance Characteristics of a Helical Rotary Screw Air-Cooled Chiller Operating Over a Range of Refrigerant Charge Conditions”, ASHRAE Trans. 1998 104(2), expressly incorporated herein by reference. Therefore, by correctly selecting the “size” (e.g., cooling capacity) of the chiller, efficiency is enhanced. Typically the chiller capacity is determined by the maximum expected design load, and thus for any given design load, the quantity of refrigerant charge in a typical design is dictated. Therefore, in order to achieve improved system efficiency, a technique of modulation recruitment is employed, in which one or more of a plurality of subsystems are selectively activated depending on the load, to allow efficient design of each subsystem while permitting a high overall system load capacity with all subsystems operational. See, Trane “Engineer's Newsletter” December 1996, 25(5):1-5. Another known technique seeks to alter the rotational speed of the compressor. See, U.S. Pat. No. 5,651,264, expressly incorporated herein by reference. It is also possible to control compressor speed using an electronic motor control, or system capacity, by restricting refrigerant flow into the compressor.
Chiller efficiency generally increases with chiller load. Thus, an optimal system seeks to operate system near its rated design. Higher refrigerant charge level than the nominal full level, however, results in deceased efficiency. Further, chiller load capacity sets a limit on the minimum refrigerant charge level. Therefore, it is seen that there exists an optimum refrigerant charge level for maximum efficiency. As stated above, as oil level increases in the evaporator, it both displaces refrigerant and has an independent effect on system efficiency.
Systems are available for measuring the efficiency of a chiller, i.e., a refrigeration system which cools water or a water solution, such as brine. In these systems, the efficiency is calculated based on Watt-hours of energy consumed (Volts×Amps×hours) per cooling unit, typically tons or British Thermal Unit (BTU) (the amount of energy required to change the temperature of one British ton of water 1° C.). Thus, a minimal measurement of efficiency requires a power meter (timebase, voltmeter, ammeter), and thermometers and flowmeters for the inlet and outlet water. Typically, further instruments are provided, including a chiller water pressure gage, gages for the pressure and temperature of evaporator and condenser. A data acquisition system processor is also typically provided to calculate the efficiency, in BTU/kWH.
U.S. Pat. Nos. 4,437,322; 4,858,681; 5,653,282; 4,539,940; 4,972,805; 4,382,467; 4,365,487; 5,479,783; 4,244,749; 4,750,547; 4,645,542; 5,031,410; 5,692,381; 4,071,078; 4,033,407; 5,190,664; and 4,747,449, expressly incorporated herein by reference, relate to heat exchangers and the like.
There are a number of known methods and apparatus for separating refrigerants, including U.S. Pat. Nos. 2,951,349; 4,939,905; 5,089,033; 5,110,364; 5,199,962; 5,200,431; 5,205,843; 5,269,155; 5,347,822; 5,374,300; 5,425,242; 5,444,171; 5,446,216; 5,456,841; 5,470,442; 5,534,151; and 5,749,245, expressly incorporated herein by reference. In addition, there are a number of known refrigerant recovery systems, including U.S. Pat. Nos. 5,032,148; 5,044,166; 5,167,126; 5,176,008; 5,189,889; 5,195,333; 5,205,843; 5,222,369; 5,226,300; 5,231,980; 5,243,831; 5,245,840; 5,263,331; 5,272,882; 5,277,032; 5,313,808; 5,327,735; 5,347,822; 5,353,603; 5,359,859; 5,363,662; 5,371,019; 5,379,607; 5,390,503; 5,442,930; 5,456,841; 5,470,442; 5,497,627; 5,502,974; 5,514,595; and 5,934,091, expressly incorporated herein by reference. Also known are refrigerant property analyzing systems, as shown in U.S. Pat. Nos. 5,371,019; 5,469,714; and 5,514,595, expressly incorporated herein by reference.